Technical field
[0001] The present invention relates to an IrDA (infrared data association) modulation/demodulation
integrated circuit device as is used in a portable telephone to achieve infrared data
communication,
Background art
[0002] A conventional IrDA modulation/demodulation integrated circuit device will be described
with reference to Fig. 5. A portable telephone 50 incorporates a telephone CPU (central
processing unit) 52 for controlling the entire telephone, a base-band integrated circuit
device 55 for processing a base-band signal, and other components.
[0003] In addition, to achieve IrDA-complying infrared data communication with an external
personal computer 60 or the like, the portable telephone 50 also incorporates an IrDA
modulation/demodulation integrated circuit device 58. For example, the IrDA modulation/demodulation
integrated circuit device 58 is used to transfer facsimile data stored in the personal
computer 60 to the portable telephone 50 by infrared rays so as to allow the thus
transferred data to be further transferred from the portable telephone 50 to a remote
location by radio waves. Of course, it is also possible to perform communication simply
between the portable telephone 50 and the personal computer 60.
[0004] The IrDA modulation/demodulation integrated circuit device 58 modulates a signal
that is going to be transmitted by infrared rays, and feeds the modulated signal to
an analog front end 4. The analog front end 4, by radiating infrared rays from a light-emitting
diode or the like, transfers the signal to the personal computer 60 or the like as
indicated by the arrow A. When, in return, the signal transferred from the personal
computer 60 by infrared rays as indicated by the arrow B is received by a photodiode
or the like provided in the analog front end 4, the analog front end 4 first performs
waveform shaping on the received signal, and then feeds the signal to the IrDA modulation/demodulation
integrated circuit device 58. The IrDA modulation/demodulation integrated circuit
device 58 then demodulates the signal.
[0005] However, the CPU 52, the integrated circuit device 55, and the integrated circuit
device 58 use clocks of different frequencies, and thus require provision of separate
crystal resonators (oscillators) 51, 54, and 57. Consequently, the CPU 52, the integrated
circuit device 55, and the integrated circuit device 58 require provision of separate
oscillation circuits 53, 56, and 59. Although not shown, the portable telephone 50
may even incorporate a further integrated circuit device that requires provision of
a separate resonator of its own.
[0006] The clock originating from the resonator 54 has a frequency of, for example, 12.6
MHz, 12.8 MHz, or 14.4 MHz. This clock is used by the CPU (not shown) provided within
the base-band IC 55 and others. On the other hand, the clock used in the IrDA modulation/demodulation
integrated circuit device 58 has a frequency of, for example, 153.6 kHz, 3.6864 MHz,
or 7.3728 MHz. These values are based on the fact that IrDA standard baud rate is
9.6 kbps, and are thus set equal to a whole number times 9.6 kHz to allow a clock
having a frequency of 9.6 kHz to be produced easily by frequency division.
[0007] In this way, the CPU 52, the IC 55, and the integrated circuit device 58 use separate
clocks, and thus require provision of separate resonators 51, 54, and 57. That is,
incorporating the IrDA modulation/demodulation integrated circuit device 58 into the
portable telephone 50 leads to an increase in the number of resonators that need to
be incorporated therein. Thus, the conventional portable telephone 50 described above
demands comparatively high cost, and requires a circuit board having a comparatively
large area to allow provision of those resonators.
Disclosure of the invention
[0008] An object of the present invention is to provide an IrDA modulation/demodulation
integrated circuit device that, by reducing the number of resonators incorporated
in a portable telephone, helps reduce the cost thereof and reduce the area of the
circuit board provided therein.
[0009] To achieve the above object, according to a first configuration of the present invention,
an IrDA modulation/demodulation integrated circuit device designed to be incorporated
in a portable telephone is provided with a PLL circuit that receives a clock used
in a base-band integrated circuit device for processing a base-band signal and that
converts the frequency of the received clock to produce a clock for the IrDA modulation/demodulation
integrated circuit device.
[0010] According to this configuration, the IrDA modulation/demodulation integrated circuit
device takes in the clock (having a frequency of, for example 12.6 MHz) used in the
base-band integrated circuit device, and converts its frequency to, for example, 7.3728
MHz by using the PLL circuit. The IrDA modulation/demodulation integrated circuit
device modulates or demodulates a signal by using the clock that has undergone such
conversion.
[0011] According to a second configuration of the present invention, in the IrDA modulation/demodulation
integrated circuit device of the first configuration described above, the PLL circuit
is provided with a first frequency division circuit for dividing the frequency of
the clock used in the base-band integrated circuit device by a factor of n (where
n is a whole number), a phase comparator for detecting the phase difference between
the output of the first frequency division circuit and the output of a second frequency
division circuit, a low-pass filter for eliminating a high-frequency component from
the output of the phase comparator, and a voltage-controlled oscillator whose oscillation
frequency is controlled by the output of the low-pass filter. In addition, a control
means is provided that divides the output of the voltage-controlled oscillator by
a factor of m (where m is a whole number) by using the second frequency division circuit
and that can select a specific value of n from among one or more values and a specific
value of m from among one or more values.
[0012] According to this configuration, even if the frequency of the clock varies from one
base-band integrated circuit device to another, it is possible to keep constant the
frequency of the clock produced by the PLL circuit by selecting appropriate values
of n and m by using the control means. The control means is realized, for example,
by controlling selectors by using control registers. In accordance with the base-band
clock frequency, a selector switches signal paths in such a way as to vary the value
of n in the first frequency division circuit. A similar selector is provided also
on the input side of the second frequency division circuit so as to vary the value
of m.
[0013] According to a third configuration of the invention, in an IrDA modulation/demodulation
integrated circuit device of the second configuration described above, the control
means selects values of n and m that satisfy a relation

(where k is a whole number) when the clock of the base-band signal has a frequency
of 12.6 MHz, selects values of n and m that satisfy a relation

when the clock of the base-band signal has a frequency of 12.8 MHz, and selects values
of n and m that satisfy a relation

when the clock of the base-band signal has a frequency of 14.4 MHz
[0014] According to this configuration, whichever frequency, namely 12.6 MHz, 12.8 MHz,
or 14.4 MHz, the clock of the base-band integrated circuit device may have, by selecting
values of m and n that satisfy one of the above-noted relations, it is possible to
obtain from the PLL circuit a clock having a frequency, for example 7.3728 MHz, that
is equal to 115.2 kHz multiplied by k.
[0015] According to a fourth configuration of the present invention, in an IrDA modulation/demodulation
integrated circuit device of the second or third configuration described above, the
IrDA modulation/demodulation integrated circuit device is connected to an analog front
end that emits and senses infrared rays, and the IrDA modulation/demodulation integrated
circuit device starts operating by being started by a starting circuit provided therein
when the analog front end senses the infrared rays.
[0016] According to this configuration, when a personal computer or the like attempts to
communicate with the portable telephone by infrared rays, it first performs a discovery
communication process in which it outputs a signal searching for a communication partner
at regular intervals. When the analog front end senses this signal, it feeds the signal
to the IrDA modulation/demodulation integrated circuit device. Designed to be able
to refer to this signal, the starting circuit then starts the operation of the entire
IrDA modulation/demodulation integrated circuit device. In this way, the portable
telephone starts communicating with the personal computer or the like.
[0017] According to a fifth configuration of the invention, an IrDA modulation/demodulation
integrated circuit device designed to be incorporated in a portable telephone is provided
with a PLL circuit that receives a clock used in a base-band integrated circuit device
for processing a base-band signal and that converts the frequency of the received
clock, a frequency division circuit for dividing the frequency of the clock output
from the PLL circuit, a reception circuit for demodulating an IrDA-complying signal
fed thereto from the outside by infrared rays by using the clock output from the frequency
division circuit, and a control register for changing the frequency division factor
of a frequency division circuit provided within the PLL circuit when the reception
circuit fails to demodulate the IrDA-complying signal.
[0018] According to this configuration, the clock produced by the PLL circuit after the
entire integrated circuit device has been started is subjected to frequency division
by the frequency division circuit and then fed to the reception circuit. If the reception
circuit fails to perform communication at that frequency, it notifies the control
register of the failure of reception. The control register is a register for setting
the frequency division factor of the frequency division circuit of the PLL circuit,
and, if reception fails, it changes the frequency division factor of the PLL circuit
and thereby changes the clock frequency. If reception succeeds, the setting made by
the control register is maintained to keep constant the frequency of the clock output
from the PLL circuit; if reception fails again, the control register further changes
the clock frequency.
Brief description of drawings
[0019]
Fig. 1 is a block diagram of a first embodiment of the present invention.
Fig. 2 is a block diagram of the PLL circuit employed in the first embodiment.
Fig. 3 is a block diagram of the transmission/reception circuit employed in the first
embodiment.
Fig. 4 is a block diagram of a second embodiment of the present invention.
Fig. 5 is a block diagram of a portable telephone employing a conventional IrDA modulation/demodulation
integrated circuit device.
Best mode for carrying out the invention
〈First Embodiment〉
[0020] A first embodiment of the present invention will be described with reference to Figs.
1 and 2. Fig. 1 is a block diagram showing a base-band integrated circuit device 2
and an IrDA modulation/demodulation integrated circuit device 3 incorporated in a
portable telephone, together with relevant components provided around them, and a
computer 60 that communicates with the portable telephone. The portable telephone
also incorporates a telephone CPU 52 and other components (see Fig. 5). An analog
front end 4 emits and senses infrared rays. When the analog front end 4 senses infrared
rays, it first performs waveform shaping on the obtained signal and then outputs it.
The analog front end 4 exchanges signals with the IrDA modulation/demodulation integrated
circuit device on a digital basis.
[0021] The base-band integrated circuit device 2 processes a base-band signal by performing
sound encoding/decoding operations, time-division multiplex operations, and other
operations. The base-band integrated circuit device 2 and the crystal resonator 1
are the same as the base-band integrated circuit device 55 and the resonator 54 incorporated
in the portable telephone 50 (see Fig. 5).
[0022] The clock produced in the base-band integrated circuit device 2 by using the resonator
1 (see fig. 1) is used to process a base-band signal, and therefore there is rather
a limited choice of frequencies as its frequency.
[0023] The IrDA modulation/demodulation integrated circuit device 3 is provided with a CPU
12, a ROM (read-only memory) 11 for storing software and the like, a RAM (random-access
memory) 10 used for temporary storage of data and the like, a logic section 8 composed
of logic devices, a transmission/reception circuit 6 for modulating/demodulating a
signal, a buffer 7 for starting the entire integrated circuit device 3 when a signal
is fed from the analog front end 4 to the transmission/reception circuit 6 while the
IrDA modulation/demodulation integrated circuit device 3 is off, and a PLL (phase-locked
loop) 5 for producing a clock signal.
[0024] The software stored in the ROM 11 is used to perform communication parameter setting
operations, signal conversion operations, and other operations in IrDA-complying data
communication. This software allows the CPU 12 to perform necessary operations. Meanwhile,
the RAM 10 is used for temporary storage of data and the like.
[0025] The IrDA modulation/demodulation integrated circuit device 3 takes in a clock signal
from the base-band integrated circuit device 2. The IrDA modulation/demodulation integrated
circuit device 3 converts the frequency of this clock by using the PLL circuit S so
as to produce a clock CL having a frequency of, for example, 7.3728 MHz. The logic
section 8 is provided with a control register 9 so that it can set the frequency division
factor of the PLL circuit 5 and thereby control the frequency of the clock CL.
[0026] According to IrDA 1.0 (version 1.0), the maximum baud rate for communication is 115.2
kbps. Therefore, as long as the frequency of the clock CL output from the PLL circuit
5 is equal to a whole number times 115.2 kHz, the IrDA modulation/demodulation integrated
circuit device 3 can obtain a signal that is in synchronism with the baud rate easily
by performing frequency division on the clock CL.
[0027] Fig. 2 is a Hock diagram showing in more detail the configuration of the PLL circuit
5. The clock used in the base-band integrated circuit device 2 (see Fig. 1) is fed
to the PLL circuit 5, where the clock is first fed to a terminal 40 of a selector
20. In the selector 20, one of three terminals 41 to 43 is selected by the control
register 9 as the destination to which to feed the clock.
[0028] For example, when the clock has a frequency of 12.6 MHz, the clock is fed to the
terminal 41; when the clock has a frequency of 12.8 MHz, the clock is fed to the terminal
42; when the clock has a frequency of 14.4 MHz, the clock is fed to the terminal 43.
Thus, the frequency of the clock fed out via the terminals 41 to 43 is f
1 = 12.6 MHz, f
2 = 12.8 MHz, and f
3 = 14.4 MHz, respectively. Moreover, when the control register 9 switches the selector
20, it simultaneously switches also a selector 27 so that different signal paths are
used in accordance with the clock frequency.
[0029] The frequency division circuit 21 provided in the stage following the selector 20
divides the frequency of the clock by a factor of n. The frequency division circuit
21 uses different values of n for the three frequencies f
1 to f
3. The signal that has undergone frequency division is then fed to a phase comparator
22. The phase comparator 22 detects a phase difference between the signal output from
the frequency division circuit 21 and the signal output from a frequency division
circuit 25. A low-pass filter 23 eliminates a high-frequency component from the output
of the phase comparator 22, and feeds the resulting signal to a voltage-controlled
oscillator 26.
[0030] The voltage-controlled oscillator 26 outputs the clock CL while varying the oscillation
frequency fo in such a way as to decrease the above-mentioned phase difference. Moreover,
the clock CL is fed, by way of one of signal paths selected by the selector 27, to
the frequency division circuit 25. In the frequency division circuit 25, the frequency
of the clock is divided by a factor of m (where m is a whole number), using the corresponding
value of m among those set for the individual signal pats. Thereafter, the clock is
fed to the phase comparator 22.
[0031] The switching of the selectors 20 and 27 is controlled by the control register 9.
For example, the control register 9 selects whole numbers n and m that satisfy a relation

when the clock frequency of the base-band integrated circuit device 2 is 12.6 MHz,
selects whole numbers m and n that satisfy a relation

when the clock frequency is 12.8 MHz, and selects whole numbers m and n that satisfy
a relation

when the clock frequency is 14.4 MHz.
[0032] Since there is a limited choice of frequencies as the frequency of the clock used
in the base-band integrated circuit device 2, by adopting a configuration as shown
in Fig. 2 that allows selection of a frequency division factor by the use of the selectors
20 and 27, it is possible to cope with many types of base-band integrated circuit
device 2. However, the clock frequency used in other integrated circuit devices and
the like, for example the telephone CPU 52 (see Fig. 5), is not standardized. Thus,
the selectors 20 and 27 cannot cope with so many types of telephone CPU 52 or the
like. In addition, there is a possibility that the clock frequency of a CPU 52 or
the like will be changed to obtain better performance or the like.
[0033] In the base-band integrated circuit device 2, since there is a limited choice of
frequencies as the clock frequency, simply by designing the selector 20 to cope with
three frequencies, it is possible to cope with many types of base-band integrated
circuit device 2. Moreover, since the base-band integrated circuit device 2 is for
processing a base-band signal, it is unlikely that the clock frequency used therein
is changed so often.
[0034] Back in Fig. 1, the analog front end 4 emits and senses infrared rays, and performs
bi-directional data communication with, for example, the personal computer 60 as indicated
by the arrows A and B. To minimize electric power consumption, the IrDA modulation/demodulation
integrated circuit device 3 is kept in an off state when no infrared communication
is taking place. When the portable telephone starts infrared communication with the
personal computer 60, the telephone CPU 52 (see Fig. 5) starts the IrDA modulation/demodulation
integrated circuit device 3 to start communication.
[0035] On the other hand, when the personal computer 60 requests the portable telephone
to start communication, according to IrDA, in the discovery communication process,
the personal computer 60 outputs a signal searching for a communication partner. When
the analog front end 4 senses this signal, it feeds the signal to the transmission/reception
circuit 6 of the IrDA modulation/demodulation integrated circuit device 3. Designed
to be able to refer to this signal even if the integrated circuit device 3 is in an
off state, the buffer 7 then starts the entire IrDA modulation/demodulation integrated
circuit device 3.
[0036] As a result, the PLL circuit 5 starts producing the clock CL, and therefore, when
the call-up signal from the personal computer 60 is sensed next time, the IrDA modulation/demodulation
integrated circuit device 3 can respond to it. In this way, the portable telephone
and the personal computer 60 start communicating with each other. In the discovery
communication process, the personal computer 60 cannot proceed to the next communication
process unless it receives a response, and therefore, even if the portable telephone
takes a little while to respond to the signal from the personal computer 60, no serious
problem will arise. After the IrDA modulation/demodulation integrated circuit device
3 is started, data communication is started through communication processes such as
negotiation. When data communication is complete, the IrDA modulation/demodulation
integrated circuit device 3 is turned off.
[0037] As described above, in this embodiment, the IrDA modulation/demodulation integrated
circuit device 3 can read in the clock signal from the base-band integrated circuit
device 2 and convert the frequency of the clock by using the PLL circuit 5. This eliminates
the need to provide a separate resonator for the IrDA modulation/demodulation integrated
circuit device 3. As a result, it is possible to reduce the number of resonators incorporated
in the portable telephone, and thus reduce the cost thereof. Moreover, with less resonators,
it is possible to reduce the area of the circuit board, and thus achieve miniaturization
of the portable telephone as a whole. Furthermore, the IrDA modulation/demodulation
integrated circuit device 3 is started automatically when infrared communication is
going to be performed, and it is kept in an off state when no infrared communication
is taking place. This helps minimize the electric power consumption of the portable
telephone.
[0038] The selectors 20 and 27 are controlled by the control register 9; however, it is
also possible to select the signal paths by securing direct connections in the selectors
20 and 27 by the use of diodes or the like in accordance with the clock frequency
of the base-band integrated circuit device 2. Moreover, for the frequency division
circuits 21 and 25, specific values of n and m are predetermined on a circuit basis;
however, it is also possible to set the values of n and m on a software basis.
[0039] The clock CL does not necessarily have to have a frequency of 7.3728 MHz but may
have a different frequency. Since the maximum baud rate according to IrDA 1.0 is 115.2
kbps, by setting the frequency of the clock CL equal to a whole number times 115.2
kHz, it is possible to obtain a clock that is in synchronism with one of the IrDA-complying
baud rates easily by frequency division. Here, for the frequency division circuits
21 and 25, values of n and m, respectively, are set that satisfy relations

,

, and

(where k is a whole number), of which appropriate values are selected by the control
register 9.
[0040] By increasing the number of signal paths that can be selected by the selectors 20
and 27, it is possible to increase the number of frequencies that the IrDA modulation/demodulation
integrated circuit device 3 can cope with as the frequency of the clock fed thereto
from the base-band integrated circuit device 2. The IrDA modulation/demodulation integrated
circuit device 3 does not necessarily have to be dedicated to IrDA-complying modulation/demodulation
functions but may serve other functions of the portable telephone.
〈Second Embodiment〉
[0041] A second embodiment of the present invention will be described with reference to
Fig. 3. As compared with the IrDA modulation/demodulation integrated circuit device
3 (see Fig. 1) of the first embodiment described above, the IrDA modulation/demodulation
integrated circuit device 3a of this embodiment is additionally provided with a function
that allows selection as performed by the selectors 20 and 27 (see Fig. 2) to be performed
automatically by a control register 33. A frequency division circuit 31 divides the
frequency of the clock CL of the PLL circuit 5 and feeds the resulting clock to a
reception circuit. In Fig. 3, such elements as are found also in Fig. 1 are identified
with the same reference numerals and symbols, and overlapping descriptions will not
be repeated.
[0042] As described previously, when the personal computer 60 (see Fig. 1) or the like attempts
to start IrDA-complying infrared communication with the portable telephone, first,
the personal computer 60 performs a discovery communication process in which it transmits,
at a baud rate of 9.6 kbps, a signal for calling up the portable telephone. If no
response is received from the portable telephone, the personal computer 60 cannot
proceed to the next communication process.
[0043] Fig. 4 shows a block diagram of an example of the transmission/reception circuit
32. When the transmission/reception circuit 32 receives a signal from the analog front
end 4 while it is in an off state, the signal is fed through a diode D and a resistor
R to a capacitor C so as to accumulate electric charge in the capacitor C. One end
of the capacitor C is grounded. When the electric charge accumulated in the capacitor
C reaches a predetermined level, the buffer 7 produces an output that causes the entire
integrated circuit device 3a to start operating. In addition, also within the transmission/reception
circuit 32, a transmission section 35 and a reception section 36 are started by applying
thereto a voltage by using switching transistors or the like.
[0044] The transmission section 35 reads in a signal from the logic section 8, performs
modulation on the signal, and then feeds it to the analog front end 4. The reception
section 36 demodulates the signal fed from the analog front end 4, and feeds the demodulated
signal to the logic section 8. If reception fails, the reception section 36 notifies
the control register 33 of the failure of reception.
[0045] In Fig. 3, when the analog front end 4 receives a signal from the personal computer
60, the transmission/reception circuit 32 feeds a signal to the buffer 7 to instruct
it to start the IrDA modulation/demodulation integrated circuit device 3a. The PLL
circuit 5 takes in the clock from the base-band integrated circuit device 2 (see Fig.
1), converts the frequency of the clock in accordance with the initial settings of
the control register 33, and then outputs the resulting clock.
[0046] If the selection made by the control register 33 matches with the clock frequency
of the base-band integrated circuit device 2, the clock CL output from the PLL circuit
5 has a frequency of, for example, 7.3728 MHz. At this time, the clock CL is subjected
to frequency division by the frequency division circuit 31 to become a clock having
a frequency equal to a whole number times 9.6 kHz. This clock having a frequency equal
to a whole number limes 9.6 kHz is fed to the reception circuit 32.
[0047] On the basis of this signal, the transmission/reception circuit 6 performs demodulation
of the signal from the analog front end 4 and other operations. However, with the
initial settings of the control register 33, the PLL circuit 5 does not always output
a clock having a frequency of 7.3728 MHz. Accordingly, the transmission/reception
circuit 6 feeds a signal indicating whether reception has been successful or not to
the control register 33.
[0048] When reception succeeds, the control register 33 maintains the settings of the control
register 33. By contrast, when reception fails, the control register 33 instructs
the selectors 20 and 27 to make different selections. As a result, the PLL circuit
5 changes the frequency of the clock CL it produces, and reception is attempted again.
[0049] The control register 33 repeats such switching until the frequency of the clock CL
becomes equal to 7.3728 MHz. If reception succeeds, the control register 33 responds
to the personal computer 60 at 9.6 kbps. The control register 33 simply switches among
the three signal paths by using the selectors 20 and 27 as shown in Fig. 2, and thus
can start communication in response to a call-up from the personal computer 60 without
undue delay.
[0050] As described above, in this embodiment, the IrDA modulation/demodulation integrated
circuit device 3a can automatically cope with different frequencies of the clock of
the base-band integrated circuit device 2 (see Fig. 1). Therefore, when the IrDA modulation/demodulation
integrated circuit device 3a is incorporated into the portable telephone, there is
no need to secure direct connections by the use of diodes or the like. As a result,
regardless of the type of the base-band integrated circuit device 2, the IrDA modulation/demodulation
integrated circuit device 3a can automatically produce a clock having a frequency
of 7.3728 MHz. A clock CL having a different frequency is produced automatically in
the same manner.
Industrial applicability
[0051] As described above, according to the present invention, there is no need to provide
a separate resonator for an IrDA modulation/demodulation integrated circuit device,
and thus it is possible to reduce the number of resonators incorporated in a portable
telephone. Accordingly, it is possible to reduce the cost of the portable telephone.
Moreover, it is also possible to reduce the area of the circuit board and thereby
achieve miniaturization of the portable telephone.
[0052] Since there is rather a limited choice of clock frequencies as the clock frequency
of a base-band integrated circuit device, by making selections in accordance with
the clock frequency, it is possible to obtain a clock having a fixed frequency from
a PLL circuit. Thus, to cope with different clock frequencies of the base-band integrated
circuit device, there is no need to prepare different IrDA modulation/demodulation
integrated circuit devices, but instead a single IrDA modulation/demodulation integrated
circuit device according to claim 2 serves the purpose.
[0053] In most cases, the clock frequency of the base-band integrated circuit device is
12.6 MHz, 12.8 MHz, or 14.4 MHz. On the other hand, the maximum baud rate according
to IrDA 1.0 is 115.2 kbps. Accordingly, if the PLL circuit produces a clock having
a frequency equal to a whole number times 115.2 kHz, it is possible to obtain a clock
that is in synchronism with one of the IrDA-complying baud rates easily by frequency
division achieved by the use of the IrDA modulation/demodulation integrated circuit
device. By subjecting a clock signal having a frequency of 115.2 kHz to frequency
division, it is possible to produce a clock signal that is in synchronism with any
one of the IrDA-complying baud rates. According to IrDA 1.1 (version 1.1), baud rates
of 0.576 Mbps, 1.152 Mbps, and 4 Mbps are used; in this case, it is necessary to produce
clocks having frequencies corresponding to those baud rates.
[0054] Moreover, at the start of data communication, the IrDA modulation/demodulation integrated
circuit device automatically starts operating. By keeping the IrDA modulation/demodulation
integrated circuit device in an off state when no communication is taking place, it
is possible to reduce the electric power consumption of the portable telephone.
[0055] Moreover, it is possible to start communication by automatically switching the frequency
division factor by the use of a control register in accordance with the clock of the
base-band integrated circuit device. There is no need to make special settings in
accordance with the type of the base-band integrated circuit device, and therefore
it is easy to incorporate the IrDA modulation/demodulation integrated circuit device
into a portable telephone.
[0056] Offering advantages as described above, an IrDA modulation/demodulation integrated
circuit device according to the present invention is suitable for use in a portable
telephone.